In Java, Garbage Collection (GC) plays a crucial role in managing memory allocation and freeing up memory that is no longer needed by the program. The default GC algorithm in Java is the Parallel GC, which can cause significant pauses in large-scale applications.

To address this issue, the Shenandoah GC algorithm was introduced in Java 12 as an experimental feature and became fully supported in Java 15. Shenandoah GC aims to minimize the pause times experienced during garbage collection, especially in applications that require low latency and high throughput.

How does Shenandoah GC work?

Shenandoah GC is a concurrent garbage collector, which means it performs the garbage collection concurrently with the running Java application. It achieves this by using the Optimistic Heap Decommit technique. When objects are allocated, Shenandoah GC marks them as “zombies” that are not yet fully committed to the heap. This allows the application to continue executing without being interrupted by garbage collection pauses.

During the concurrent marking phase, Shenandoah GC identifies all live objects and updates their references. To ensure consistency, it performs multiple iterations of the concurrent marking phase, keeping the resulting pause durations low. Once the marking phase is completed, Shenandoah GC reclaims the garbage objects and frees up the memory.

Impact on memory allocation performance

One of the key advantages of Shenandoah GC is its impact on memory allocation performance. Since Shenandoah GC performs the garbage collection concurrently, it significantly reduces the pause times experienced by the application. This is especially beneficial for latency-sensitive applications, such as real-time systems or interactive applications that cannot afford noticeable pauses.

By minimizing pause times, Shenandoah GC allows Java applications to make better use of available resources and provide a smoother user experience. It also improves overall throughput, as the application can continue executing while garbage collection is taking place in the background.

Enabling Shenandoah GC

To enable Shenandoah GC in your Java application, you need to use the following JVM options:

-Xmx<max_memory_allocation> -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC

Replace <max_memory_allocation> with the desired maximum memory allocation for your application, such as 4g for 4 gigabytes.

Conclusion

Shenandoah GC brings significant improvements to memory allocation performance in Java applications. By reducing pause times and performing concurrent garbage collection, it improves the overall responsiveness and throughput of the application, making it a valuable option for latency-sensitive scenarios.

To take advantage of Shenandoah GC, ensure that you are using the supported Java version and enable it using the appropriate JVM options. Monitoring the performance of your application with Shenandoah GC enabled will help you measure its impact and assess its benefits in your specific use case.